Gas turbine engines typically comprise parts, such as turbine blades having complex curved surfaces. Such blades comprise an airfoil providing an aerodynamic shape and a root providing a fixing (or firtree) for assembly with turbine discs. When machining the firtree, the airfoil needs to be precisely positioned in the coordinate system of computer numerical control (CNC) machines.
In conventional designs, the blades are secured during firtree machining to holding fixtures or encapsulation blocks, which have contact features for clamping on the CNC machines. The contact features function as positioning datum relative to which the airfoil is to be precisely positioned. Any manufacturing inaccuracy in the encapsulation or the holding fixture can cause deviation in the measurement of distances between the contact features and a plurality of control points on the airfoil. As a result, rework of encapsulation blocks or fine adjustment of the holding fixtures may be required, which may increase the risk of blade scraps.
One known method for correcting inaccuracies is to determine the position and orientation of the airfoil by capturing the entire surface of the airfoil. This may be done by measuring numerous discrete points on the blade's curved surface using contact or non-contact measurement. The resulting point grids or point clouds are then compared to a three-dimensional computer-aided design (CAD) model of the part. Any discrepancies with the model may be corrected by applying a variety of treatment methods. However, these solutions prove cumbersome and ineffective for a variety of applications. In particular, contact probing measurement of a large amount of points is inefficient for high-volume production while non-contact measurement, which is less accurate than probing measurements, is affected by lighting conditions.
There is therefore a need for an improved system and method for positioning error compensation during manufacturing of complex-shaped gas turbine engine parts.